Chemical mechanical polishing (sometimes known in the art as chemical mechanical planarization), or CMP, is in general a well-known process used in the fabrication of semiconductor devices. CMP combines mechanical polishing (using, for example, abrasive slurries) with selective chemical reactions to increase the mechanical removal rate of material. The chemical reaction(s) particularly provide greater material removal selectivity than mechanical polishing alone.
CMP is commonly used to flatten the surface of a wafer after etch and/or deposition steps, generally to such a degree that subsequent photolithography steps have a sufficient focus margin.
In general, CMP is performed by using a polishing pad in combination with a slurry of water, abrasives, and reactive chemicals for the desired chemical reaction or reactions. The polishing pad is caused to be pressed against the wafer surface and relative motion between the wafer and the pad is imparted (that is, by moving one or both of the wafer and the pad).
The polishing pad is conventionally a porous pliable material. Polyurethane foam is particularly common for use as a polishing pad. Surface asperities of the polishing pad are critical to the polishing process because they provide the mechanical polishing action. However, as the pad is used for polishing, it tends to become smoother as the asperities are rubbed away and/or as slurry residues build up in the pores. As a result, the polishing process is degraded. It is therefore conventionally known to condition the polishing pad to roughen the surface and increase the open porosity of the foam.
FIGS. 1 and 2 are a side elevational and a perspective view, respectively, of a conventional CMP pad conditioning apparatus 10. As generally shown, an abrasive member, such as a member 12 coated with an abrasive, such as diamond, is pressed, relatively, against a surface of the polishing pad (not illustrated) with a certain force. The member 12 and the surface of the polishing pad are then moved relative to one another.
For example, abrasive member 12 is mounted on an arm 14 (by way of a mount or support 20) in a known manner. The arm 14 may be in turn pivotably mounted in a known manner such that the arm 14 can be raised or lowered so as to press a surface of member 12 against a surface of a polishing pad as desired. The arc labeled R in FIGS. 1 and 2 indicate this motion of arm 14 about an axis 17.
An alternative known system (not illustrated) does not pivot but instead applies a vertical pressing force by way of support 20, such as by extending and retracting support 20 along its axis.
As seen in FIGS. 1 and 2, arm 14 may be also laterally rotatable about an axis 16 so that the arm 14 (and the member 12 mounted thereon) can sweep out an arc of motion relative to a polishing pad surface. The lateral sweeping motion of arm 14 may be powered, such as by a conventional motor (not shown) connected to shaft 18.
As mentioned above, arm 14 is preferably mounted at shaft 18 in a manner permitting an opposite end of arm 14 to translate vertically. In particular, this vertical translation permits member 12 to be lowered into contact with a polishing pad. The connection between arm 14 and shaft 18 is any standard arrangement permitting the required motion about axis 17, for example and without limitation, a hinge pin or a bushing/shaft assembly.
Arm 14 may be raised and lowered by any conventional means, including without limitation, manual and mechanical means (the latter not being shown).
In addition, the member 12 is mounted on the arm 14 by way of a mount or support 20 so as to accommodate raising or lowering arm 14 relative to the horizontal while maintaining a surface of member 12 in contact with a surface of the polishing pad. For example, a conventional gimbal, hinge pin, ball and socket joint, etc. structure may be provided to mount member 12. In general, member 12 is mounted so as to be pivotable about axes 21 and 23, which motion permits all angles of orientation between abrasive member 12 and an opposing polishing pad surface to be accommodated.
At the outset, it will be appreciated that the simple resting weight of the arm 14, member 12, and member mount 20 will tend to cause arm 14 to rotate about axis 17 on shaft 18 (as indicated by the arrow R), which will correspond to a given pressing force against a polishing pad surface opposed to member 12. In an alternative known arrangement, the arm 14 does not pivot about axis 17. Instead the member mount 20 (with member 12 mounted thereon) is axially movable, and its resting weight corresponds to a given pressing force against a polishing pad opposed to member 12.
However, the relative pressing force between the member 12 and the polishing pad is a very important factor in controlling polishing pad conditioning, such that variations in the pressing force cause variations in polishing pad conditioning.
In particular, the pressing force of a conditioning assembly passively resting on a polishing pad cannot be changed to provide different conditioning results or to conform to conditioning requirements for different polishing pad materials.
Some conventional arrangements envision fixedly mounting a weight on the arm in order to provide a different pressing force than that corresponding to the passive weight of the assembly.
Also, some conventional pad conditioning arrangements use sensors, such as load cells, as part of a calibration process to determine a given pressing force. However, this still does not address the problem of adjusting the pressing force during operation. Also, the magnitude of the pressing force still cannot be altered.